The long term objective of the proposed study is focused on the purification to possible homogeneity and characterization of the glucocorticoid receptors in developing and adult rat liver. This may add information on the relationship between the structure of glucocorticoid receptors and elucidate their molecular mechanism of action in vivo and should also enable us to test directly the effect of glucocorticoid receptors on gene expression in vitro. The study of mechanisms by which glucocorticoids control a differential gene expression in developing rat liver may provide a useful approach to the understanding of the mechanism of steriod hormone action, namely an attempt to identify the factors essential for a response to the hormone that are lacking before a particular stage of development.